Water-soluble pre-ceramic polymers and methods of their production

ABSTRACT

A method of making water soluble pre-ceramic polymers includes heating hydrated metal salts above their melting points. A flow of air drives off water and most of the ligands. Polymerization occurs in the melt and the remaining side groups make the polymer soluble in water. Solubilization in water is accomplished by mixing the polymer with distilled water and heating the mixture at a temperature higher than about 60° C. Thus prepared polymer solutions may be used to form fibers, films, and composite matrices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a process of synthesizing inorganic polymers, and more particularly to the production of water-soluble aluminum and yttrium based polymers for use in thin films, fibers, and composite matrices. 2. Description of the Related Art

Ceramics can be prepared from a number of precursor states such as molten liquid, powder, vapor, and polymer. The polymer based process, despite its relatively short history, has been the subject of much interest because of its potential economic benefits. Compared to vacuum deposition techniques, it is a far more cost-effective way of depositing ceramic coatings. Antireflective coatings on window glass and silicon solar cells, planarization layers on ultrasmooth surface mirrors, and superconducting thin films on microwave cavities are some examples. In addition to the economic benefits, the technique allows fabrication of shapes that are impossible by other methods. For example, the viscoelasticity of high molecular weight polysiloxane and zirconium acetoacetonate derivatives allows the fabrication of high modulus SiO₂ and ZrO₂ fibers, respectively.

Despite the success, the polymer route has not yet gained industry-wide acceptance mainly because of the following two problems: 1) difficulty in handling, or intractibility, and 2) low ceramic yield. The intractibility arises due to the improper choice of precursors. The most often used precursors for polymer synthesis are metal alkoxides or derivatives of metal alkoxides, all of which are hygroscopic. Use of large quantities of anhydrous organic solvents and handling in inert atmospheres are therefore unavoidable. Low ceramic (or char) yield is another problem. Most pre-oxide polymers have ceramic yields far below 30% and no polymer with a yield greater than 40% has been reported. The low yields cause large drying and sintering shrinkages which invariably lead to poor dimensional control.

Therefore, it is desirable to have a process for synthesizing water-soluble pre-ceramic polymers of high char yields from inexpensive precursors. Such a process would be of tremendous economic value.

SUMMARY OF THE INVENTION

The present invention solves the aforementioned problems with the prior art as well as others by providing a method for making a water soluble pre-ceramic polymer. The water soluble pre-ceramic polymers of the present invention are suitable for impregnating preform tubes. Further impregnations are attainable beyond 18% by a unique approach with addition of alcohol.

Alternately, fibers are fabricated directly from the pre-ceramic polymer by adjustment of the polymer solution viscosity to exceed 10 ⁶ centipoise (cps).

Advantageously, several compounds including nitrate stabilized zirconia sol, acetate stabilized zirconia sol, Y₂ O₃ stabilized ZrO₂ powder, and Y₃ Al₅ O₁₂ may be added to the pre-ceramic polymer for particular applications. Also, an item of hot pressed Y₂ O₃ stabilized ZrO₂ may be coated with a highly concentrated polymer solution of the present invention.

The method of the present invention makes a water soluble pre-ceramic polymer by heating a hydrated metal salt in a furnace to a temperature above the melting point of the hydrated metal salt. A flow of air is provided above the melt to drive the polymerization reaction forward until a predetermined weight loss of the pre-ceramic polymer is obtained.

An object of the present invention is to synthesize water-soluble polymers in which major portions consist of metal and oxygen atoms.

Another object of the present invention is to synthesize water-soluble pre-ceramic polymers from inexpensive precursors.

Yet another object of the present invention is to impregnate fiber preforms with polymer solutions of the present invention as precursors to matrices.

Still another object of the present invention is to spin ceramic fibers of fine diameter and microcrystalline grains from the polymers of the present invention.

Still a further object of the present invention is to deposit thin films on ceramic substrates from the polymers of the present invention.

The various features of novelty characterized in the present invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, the operating advantages attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the present invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1a and b are photographs of an Al₂ O₃ polymer powder prepared in accordance with the present invention and on its right a solution prepared by dissolving the polymer in water;

FIG. 2a and b are pairs of x-ray diffraction patterns of a pre-Al₂ O₃ polymer plotted as a function of 2⊖, which is the angle between the transmitted and diffracted x-rays. It indicates that the as-prepared polymer is amorphous and that after heating at 1200° C. for 1 hour crystallizes to α-Al₂ O₃ ;

FIG. 3 (a) is a combined graph of a differential thermal analysis (DTA) and a thermogravimetric analysis (TGA). It indicates that the polymer crystallizes from an amorphous state to ⊖-Al₂ O₃ at 776° C. and α-Al₂ O₃ at 1082° C., respectively;

FIG. 3 (b) is another combined graph of a DTA and TGA. It shows that the polymer has a ceramic yield as high as 80% on a weight percent basis;

FIG. 4 is a plot of the viscosity in centipoise (cps) of a polymer solution as a function of polymer concentration in water;

FIG. 5 is a plot of the weight gain of a ceramic matrix composite tube as a function of the number of impregnations with a pre-Al₂ O₃ polymer solution;

FIG. 6 is a Scanning Electron Microscope microphotograph of a cross-section of an Al₂ O₃ fiber/Al₂ O₃ matrix composite;

FIG. 7 is a photograph of an Al₂ O₃ fiber being pulled from a concentrated pre-Al₂ O₃ polymer solution;

FIG. 8 (a) is a photograph of as-drawn Al₂ O₃ fibers from a polymer solution;

FIG. 8(b) is a photograph of the same fiber after heat treatment at 1200° C.;

FIG. 9(a) is an SEM micrograph of the same fiber shown in FIG. 8(b); and

FIG. 9(b) is an SEM micrograph of a fractured surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention resides in the ability of hydrated metal salts to undergo polycondensation reactions at temperatures above their melting points. The method comprises the steps of heating metal salts to temperatures above their melting points and holding the melts in a well ventilated oven to drive off hydrated water and the ligands. The constant flow of fresh air above the melt in the range of 200 to 2000 cubic centimeters per minute and preferably between 500 and 1000 cc/min drives away the ligands as they are expelled from the growing polymer chains. This drives the polymerization reaction forward and increases the lengths of the polymers. The heating is continued until all the free ligands are driven off and only solid polymers are left. This is determined by the amount of weight lost during heating. The overall process may be represented by the following chemical equation: ##STR1## where M is a metal such as aluminum or yttrium, and n is the number of repeating units in the polymer chain. The equation is not balanced for simplicity. Care must be exercised so as not to overheat the polymers. The temperature above the melting point of the polymers should not exceed 100° C. Some ligands must be left attached to the polymers as side groups for them to be water-soluble. A predetermined weight loss of less than 73 weight percent for Al₂ O₃ polymers and 36 weight percent for Y₂ O₃ polymers provides for this. The solid polymers readily dissolve in heated water at a temperature higher than about 60° C. to give highly concentrated solutions. The polymer solutions have shelf lives in excess of 6 months at room temperature and at a pH below 4.

The following examples are illustrative of test results that demonstrate the present invention:

EXAMPLE 1

Al₂ O₃ Precursor Polymer

1488 g of Al(NO₃)₃ 9H₂ O powder is spread evenly in a rectangular ceramic tray whose dimensions are approximately 12"×10.5"×2.5". The tray is placed in a large drying oven equipped with a convection fan. The temperature of the oven is raised to a temperature between 110° and 190° C. and kept at that level to allow the melting of the salt, expulsion of hydrated water and nitrate ligands, and polymerization to take place. Heating is continued for 48 hours or until the total weight loss is between 71.5% and 73% on a weight percent basis. The formed white, amorphous polymer readily dissolves in water at temperature exceeding 60° C. (FIG. 1a and b), and has a ceramic yield of 80% by weight, note FIG. 3(b), and crystallizes to fully crystalline α-Al₂ O₃ at 1080° C. as evidenced by FIGS. 2 and 3(a).

EXAMPLE 2

Y₂ O₃ Precursor Polymer

1.0 g of Y(NO₃)₃ 6H₂ O is placed in a 6 inch long Al₂ O₃ boat. The boat is then placed inside a well ventilated oven and the temperature is raised to 275° C. The temperature is maintained until the total weight loss is between 34% and 36% on a weight percent basis. The formed white, solid polymer has a ceramic yield of 40% by weight.

EXAMPLE 3

Al₂ O₃ Matrix Composite Fabrication

The same procedure as described in Example 1 is followed for the preparation of a pre-Al₂ O₃ polymer solution. The solution concentration is adjusted so that the total polymer to water weight ratio is 1:2. The viscosity of the solution is approximately 50 centipoise, refer to FIG. 4, and the density equals 1.47 g/cc. This polymer solution is used to vacuum impregnate an Al₂ O₃ preform tube whose dimensions are 1.5" I.D., 1.75" O.D., and 1" in length. The tube is prepared by filament winding Almax, a registered trademark of Mitsui Mining Co., Japan, Al₂ O₃ fibers. The impregnated tube is dried in air at 110° C. for 24 hours and calcined at 500° C. The process of impregnation and calcination is repeated 15 times with 2 intermittent (after five repeats) 1100° C. sintering cycles. The porosity of the tube after 15 impregnation cycles is 18%. It has been found that further impregnations do not decrease the porosity beyond 18%. It can be decreased further only by an alcohol addition, as described in Example 4.

EXAMPLE 4

Al₂ O₃ Matrix Composite Fabrication--Alcohol Addition

The same fabrication procedure as in Example 3 is followed to prepare a solution whose concentration based on Al₂ O₃ content is 25% by weight. 7.9 g of methanol (MeOH) is added to 160 g of this solution and stirred. The viscosity of this solution is 14.8 cps. A solution containing only water possesses a viscosity of 46.6 cps. The concentration of alcohol is only 5%, but the drastically reduced viscosity]and surface tension of the solution greatly improves the penetrability of the polymer in preforms. With the modified solution, porosity levels as low as 13% can be achieved, as shown in FIG. 5. This composite tube has a dense microstructure, as seen in FIG. 6.

EXAMPLE 5

Al₂ O₃ --ZrO₂ Matrix Composite Fabrication

The same procedure as in Example 1 is followed to prepare a pre-Al₂ O₃ polymer solution. The concentration is adjusted so that the polymer to water ratio is 1:1.75 by weight. 95 g of nitrate stabilized zirconia sol (Zr 10/20, a registered trademark of Nyacol, Inc., Ashland, Mass.) whose concentration is 20% by weight is added to 262.5 g of this solution. The formed mixture is clear and colorless. For long term stability, the solution pH must be below 2. This mixture is used to fabricate Al₂ O₃ --ZrO₂ matrix composite tubes by following the same impregnation and heat treatment cycles described in Example 3.

EXAMPLE 6

Al₂ O₃ --ZrO₂ Matrix Composite Fabrication

The same preparatory procedure as in Example 5 is followed, except for the replacement of ZrO(NO₃)xH₂ O for the nitrate stabilized sol.

EXAMPLE 7

Al₂ O₃ Fibers

The same materials and procedure as in Example 1 are followed to prepare a pre-Al₂ O₃ polymer solution. The solution is then concentrated with mild heat (60° to 80° C.) until the room temperature viscosity exceeds 10⁶ centipoise. A glass rod or stainless steel spatula is inserted into the polymer mass and quickly pulled up. Depending on the pulling speed and the viscosity of the solution, fibers between 10 and 100 μm in diameter can easily be pulled, see FIG. 7. Fiber lengths in excess of 2 m can be achieved readily. FIG. 8(a) shows as-spun fibers that are fine and flexible. The as-spun fibers are dried and calcined by heating them in an electrical box furnace at l° C./min to 600° C. From 600° C., the temperature is raised to 800° C. at 5° C./min. Following an isothermal hold at 800° C. for 1 hr., the temperature is raised to 1200° C. at 10° C./min. It is held at 1200° C. for 1 hr and then cooled to room temperature at 5° C./min. FIG. 8(b) shows the heat treated α-Al₂ O.sub. 3 fibers. It clearly shows that the heat treatment did not damage fiber integrity. FIG. 9(a) shows an SEM micrograph of a sintered α-Al₂ O₃ fiber. The 10 μm fiber is fully dense and consists of grains that are approximately 0.2 μm in diameter.

EXAMPLE 8

Al₂ O₃ --(Y₂ O₃)--ZrO₂ Fibers

The same preparatory procedure as in Example 7 is followed, except for the addition of fine Y₂ O₃ stabilized ZrO₂ powder. The powder is prepared by dispersing commercially available 8 mole % Y₂ O₃ stabilized ZrO₂ (Zircon Products, Inc., New York) in water at a pH of 4. The dispersion is covered with a polyethylene film and allowed to stand for 1 week. Particles finer than 0.2 μm in diameter are collected by classification and added to the alumina polymer solution. The fiber spinning and heat treatment procedures are exactly the same as those described in Example 7.

EXAMPLE 9

Al₂ O₃ --Y₃ Al₅ O₁₂ Fibers

The same preparatory procedure as in Example 8 is followed, except for the replacement of ZrO₂ particles with Y₃ Al₅ O₁₂. Y₃ Al₅ O₁₂ powder is prepared by dissolving 86.1774 g of Y(NO₃)₃ 6H₂ O and 140.6739 g of Al(NO₃)₃ 9H₂ O in 200 g of distilled water. In a separate beaker, 150 g of aqueous NH is mixed with 300 g of distilled water. While vigorously stirring, the first solution is poured into the ammonia solution. White, gelatinous particles precipitate immediately upon mixing. The powder suspension is then washed 4 times with excess distilled water. It is then calcined at 800° C.

EXAMPLE 10

Al₂ O₃ Thin Films

The same materials and preparatory procedure used in Example 1 are followed to produce a highly concentrated polymer solution. The polymer concentration is between 50% and 60%. A polished piece of hot pressed Y₂ O₃ stabilized ZrO₂ is then dipped into the solution and slowly pulled upward. The coated piece is then dried at room temperature for 24 hr and heated to 500° C. at l° C./min. The temperature is then raised to 1200° C. at 5° C./min and held for 1 hr. The process of dipping, drying, and heating is repeated until a sufficiently thick coating is obtained.

The foregoing examples are intended for illustrative purposes and are not meant to limit the present invention. For example, other fiber forming techniques such as gas jet fiberization or a spinnerette may be employed for making fibers in Example 7. Similarly, other coating techniques may be used in Example 10.

The following Table 1 provides several precursors tested including the weight loss and water solubility.

                  TABLE 1                                                          ______________________________________                                         PRECURSOR  WEIGHT LOSS*  WATER SOLUBILITY                                      ______________________________________                                         AlCl.sub.3 6H.sub.2 O                                                                     18.3%         Yes                                                              24.4          Yes                                                              48.0          Yes                                                              59.0          Yes                                                              61.4          Yes                                                              61.66         Yes                                                              74.7          No                                                    Al (NO.sub.3).sub.3 9H.sub.2 O                                                            63.5          Yes                                                              64.3          Yes                                                              66.5          Yes                                                              67.3          Yes                                                              68.3          Yes                                                              71.1          Yes                                                              72.6          Yes                                                              72.9          Yes                                                              73.0          No                                                               73.6          No                                                               82.9          No                                                    Y (NO.sub.3).sub.3 6H.sub.2 O                                                             14.2          Yes                                                              17.4          Yes                                                              24.5          Yes                                                              27.4          Yes                                                              35.1          Yes                                                              36.1          No                                                    3Y (NO.sub.3).sub.3 6H.sub.2 O                                                            55.8          Yes                                                   5Al (NO.sub.3).sub.3 9H.sub.2 O                                                           57.6          Yes                                                              58.6          Yes                                                              59.8          No                                                               72.8          No                                                    ______________________________________                                          *Weight loss during heat treatment above 110° C.                  

The process of the present invention is applicable to all hydrated metal salts that melt at low temperatures. For example yttria and yttrium aluminate garnet (YAG) polymers have been successfully processed by the present invention.

The pre-ceramic polymers are easily employed in any of the following commercial applications as well as others.

The low viscosity of the highly concentrated pre-ceramic polymer solutions is ideal for coating or filling micropores. Greater than 10 volume percent alumina solutions with a viscosity less than 100 centipoise have been prepared. This solution is used as a matrix precursor.

The wettability of the pre-ceramic polymer on most commercially important substrates makes it suitable for thin, functional coatings. Deposition of thin, transparent, adherent coatings on glass slides by dip coating has been demonstrated.

The pre-ceramic polymers exhibit high yield which allows little shrinkage related problems. Currently practiced processes for fiber production require either fine powder addition and/or slow heating to control shrinkage.

Binders currently employed in powder processing are fugitive organics, such as poly vinyl alcohol, which are pyrolyzed during heat treatment. Because of its high char yield and suitable rheology, the pre-ceramic polymers of the present invention act as processing aids during forming but as fillers during sintering to decrease the overall shrinkage of bulk ceramics.

Because of their molecular nature, the pre-ceramic polymers can be consolidated and partially sintered to form ultrafine pore filters.

The present invention provides the following advantages over conventionally prepared polymers:

To the best of the inventors' knowledge there are no traditionally prepared pre-oxide polymers that are water soluble.

Yield is one of the most critical features of a polymer since it has a direct consequence on near-net shaping. The pre-ceramic polymers of the present invention have oxide char yields higher than any of the previously known polymers. The alumina precursor polymer, for example, has a char yield near 80%. Prior to the present invention, the highest yield ever reported was 40%.

Because the metal salts employed for this process are readily available at low cost and the only processing equipment required is a ventilated oven, the cost of the polymer is low.

Unlike metal alkoxides, no polymerization is required for rheology control. Rheology is precisely controlled by simply adjusting the only theology controlling parameter: concentration. Such precise theology control is impossible with metal alkoxides.

While specific embodiments of the invention have been shown and described in detail to illustrate the principles of the present invention, certain modifications and improvements will occur to those skilled in the art upon reading the foregoing description. It is thus understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly in the scope of the following claims. 

We claim:
 1. A method for making a water soluble pre-ceramic polymer, comprising the steps of:heating a hydrated metal salt in a furnace to a temperature above the melting point of the hydrated metal salt, said hydrated metal salt being a member selected from the group consisting of a hydrated metal nitrate salt, and a hydrated metal chloride salt; providing a flow of air above the melted hydrated metal salt to drive a polymerization reaction forward; forming a water soluble pre-ceramic polymer; maintaining the heat at a temperature not exceeding 100° C. above a melting point of the pre-ceramic polymer and flow of air until a weight loss ranging from about 18% to 73% weight percent of the pre-ceramic polymer is obtained; and adding water to the pre-ceramic polymer to make a pre-ceramic polymer solution with a polymer to water weight ratio of at least 1 to 1.5.
 2. A method as recited in claim 1, further comprising the step of adding an alcohol to the pre-ceramic polymer solution.
 3. A method as recited in claim 1, further comprising the step of concentrating a viscosity of the pre-ceramic polymer solution to exceed about 10⁶ centipoise.
 4. A method as recited in claim 1, further comprising the step of adding a member selected from the group consisting of a nitrate stabilized zirconia sol, an acetate stabilized zirconia sol, Y₂ O₃ stabilized ZrO₂ and Y₃ Al₅ O₁₂.
 5. A method as recited in claim 1, wherein the hydrated metal salt is a member selected from the group consisting of Al(NO₃). 9H₂ O and Y(NO₃). 6H₂ O.
 6. A method as recited in claim 5, wherein the heating step includes heating Al(NO₃). 9H₂ O to a temperature between about 110° to about 190° C.
 7. A method as recited in claim 5, wherein the heating step includes heating Y(NO₃). 6H₂ O to a temperature of about 275° C.
 8. A method as recited in claim 2, wherein the adding step includes an alcohol being a member selected from the group consisting of methanol, ethanol, n-propanol, and isopropanol.
 9. A method as recited in claim 8, wherein the adding step includes the step of providing an alcohol concentration of about 5% on a weight percent basis to make a viscosity of about 14.8 cps. 